While apoptosis can be initiated by a wide variety of signals, the associated morphologic changes are invariant. This is based on the ability of diverse apoptotic signals to activate a common death pathway that is initiated by the release of apoptogenic factors from the mitochondria. At least two of these factors are involved in the activation of caspases. While most death signals require the release of factors from the mitochondria to activate the downstream caspases, signaling from death receptors can, in many cases, bypass the mitochondria. We have determined that Bcl-2/XL block cytochrome c release despite being unable to block death receptor-induced apoptosis. In contrast, loss of mitochondrial membrane potential (delta Sigma m) occurred to the same degree as in cells that cytochrome c release was observed. Maintenance of delta Sigma m required the inhibition of downstream caspases. Taken together these data suggest that Bcl-2/XL functions to control the release of apoptogenic factors while caspases mediate loss of Delta Sigma m. This suggests that in addition to targeting the cytoskeleton and genome, caspases target the mitochondria for inactivation. Based on the preliminary data presented within, we hypothesize that initiator caspases target the mitochondria to uncouple electron transport. This results in the production of reactive oxygen species. To prevent excessive production of reactive oxygen species that could be damaging to neighboring cells or the phagocyte effector caspases depolarize the mitochondria. The aims of this proposal are designed to further characterize caspase-dependent mitochondrial depolarization as well as to isolate the factor(s) responsible for loss of delta Sigma m. These results also have implications in the determination of the "point of no return" in apoptosis. We will also examine the effects of inhibiting caspase-dependent depolarization on the ability of cells to recover from a death stimulus as well as the phenotype of cells that recover. This has implications on the role of apoptosis in tumorigenesis and drug resistant cancer.